Analysis of Staphylococcal Diversity in the Skin Microbiota of Healthy Riding Horses

<b>Background:</b> In animals, staphylococci constitute a significant part of the normal skin microbiota and mucous membranes. There is limited information available on staphylococci isolated from healthy horses. These skin-associated bacteria can be easily transferred between animals and horse riders via direct contact. Patients undergoing hippotherapy (i.e., medical or therapeutic sessions with horses) are especially at risk of being colonized by horse skin-associated bacteria. However, it remains unclear whether equine skin is colonized by antimicrobial-resistant (AMR) opportunistic pathogens, which may be of concern to human health. <b>Methods:</b> We cultivate staphylococci from samples collected from healthy, non-vet-visiting horses who live on private farms in a rural area. In total, 61 strains were isolated and identified at the species level using matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). <b>Results:</b> The diversity of <i>Staphylococcus</i> species in the equine skin microbiota was relatively high and, with the exception of <i>S. aureus</i>, all the other recovered strains were coagulase-negative staphylococci (CoNS). In total, eleven different staphylococcal species were identified: <i>S. xylosus</i>, <i>S. sciuri</i>, <i>S. vitulinus</i>, <i>S. equorum</i>, <i>S. succinus</i>, <i>S. nepalensis</i>, <i>S. lentus</i>, <i>S. fleurettii</i>, <i>S. aureus</i>, <i>S. chromogenes</i>, and <i>S. simulans</i>. <b>Conclusions:</b> These results indicate that healthy equine skin is colonized by opportunistic pathogens that can be causative agents of infections that are also severe in humans. The resistance among the isolated strains was observed in eight antimicrobials of the total tested and 36% (22/61) of the isolates were resistant to at least one antimicrobial. However, their resistance to critically important antibiotics used in human medicine was low. Seven isolates (11.5%; 7/61) were classified as multidrug-resistant (MDR). <i>S. aureus</i> (1/61) showed MDR and was methicillin-resistant. The <i>S. aureus</i> isolate contained genes conferring resistance to antibiotics, i.e., β-lactams (<i>blaZ</i>, <i>mecA</i>), aminoglycosides (<i>aac</i>(<i>6</i>')/<i>aph</i>(<i>2</i>″)), and macrolide-lincosamide-streptogramin B (<i>erm</i>(<i>B</i>), <i>erm</i>(<i>C</i>), and <i>lun</i>(<i>A</i>/<i>B</i>)). Also CoNS harbored genes conferring resistance to β-lactams (<i>blaZ</i>), aminoglycosides (<i>aac</i>(<i>6</i>')/<i>aph</i>(<i>2</i>″), <i>ant</i>(<i>4</i>')<i>-Ia</i>), MLSB (<i>erm</i>(<i>B</i>), <i>erm</i>(<i>C</i>), <i>lun</i>(<i>A</i>/<i>B</i>)), and tetracycline (<i>tetK</i>, <i>tetL</i>).